Arene-perfluoroarene interactions in crystal engineering: Structural preferences in polyfluorinated tolans

The compounds 4-ROC₆F₄C=Ph (4) where R = Me (a), Et (b), Prⁿ (c), Prⁱ (d), Buⁿ (e), W-C₅H₁₁ (f), PhCH₂ (g), PhCH₂CH ₂ (h), 4-MeC₆H₄ (i), 4-EtC₆H ₄ (j) and menthyl (k), have been prepared by reaction of C ₆F₅C=CPh (3) with ROH in the presence of KOH, and characterised by NMR (¹H, ¹³C, ¹⁹F) and mass spectroscopy. The single-crystal structures of PhC=CPh·C ₆F₅C=CC₆F₅ (1:2), 3 and 4a,c,d,f,g,i have been determined by X-ray diffraction at 120-160 K, and that of 3 also calculated by an ab initio pseudo-potential DFT method. The tolan moiety is nearly planar in each molecule except 4a, which has a Ph/C ₆F₅ dihedral angle of 25.8°. The R group adopts an out-of-plane orientation except in 4f, where the n-pentyl chain is nearly coplanar with the C₆F₅ group at the cost of severe distortion of the C(ar)-C(ar)-O angles (which differ by 13.5°). The structures of 1:2, 3 (pseudo-isomorphous with 1:2), 4a, 4g (disordered) and 4i contain stacks of alternating arene and perfluoroarene moieties; 4c and 4d form discrete centrosymmetric dimers with arene/ perfluoroarene overlap, while 4f forms infinite stacks without such overlap. Differential thermal analysis and transmitted polarised light microscopy revealed no liquid-crystalline behaviour of 3 and 4.